Feathers are an important waste product of the poultry industry, with about 4 million tons being produced per year world-wide. Although minor amounts thereof find use in for instance clothing, insulation and bedding, as well as a larger amount in the preparation of feather meal for the production of animal feed, there are currently insufficient (economically interesting) applications for such large quantities of feathers. As for environmental reasons, burning or burying of feathers is not always a practical alternative; these amounts of waste feathers present a difficult disposal problem for industries such as the poultry industry.
Thus, it is a general object of the invention to provide a range of economically viable uses of waste feathers, as well as to provide processes via which waste feathers can be put to such uses.
Feathers mainly consist of a fibrous protein material called keratin. Keratins are water-insoluble and protease-resistant proteins with a molecular weight of approximately 10 kDa. As shown in FIG. 1, each keratin molecule—indicated as (A) in FIG. 1—consists of a central part (β) which forms a crystalline β-sheet, and two randomly ordered chain ends, indicated as (N) and (C) respectively. In the feather, the central parts (β) of multiple keratins are joined to form a so-called microfibril (B) of about 30 angstroms in diameter. The chain ends (C) and (N) contain inter- and intra-chain cross-links (C) in the form of disulphide bonded dimeric amino acids (cystine) and form the amorphous matrix in which the microfibril is embedded. The disulphide bonds (C) and the molecular organisation of the proteins in the feathers impart insolubility and resistance to most proteolytic enzymes.
There is already an extensive amount of prior art relating to keratins, keratin-hydrolysates and products derived from keratins, as well as to the preparation and uses thereof. Especially the cosmetic and textile industries are interested in keratin-derived products for styling and modifying hair and wool. Shampoos and nail polish with hydrolysed keratin are just examples of prior art formulations that make use of keratins. Even though these applications are numerous, they are basically product formulations using mostly hydrolysed keratin as one of the components. Such products have a low added value and are of only limited use.
Prior art where keratins are used as a polymer in films or coatings is much less abundant as is evident from a review of this art in the thesis of one of the inventors (P. Schrooyen: “Feather keratins: modification and film formation”, 1999, Thesis University of Twente, Enschede, The Netherlands). Generally, according to the processes described in these references, the insoluble keratins are extracted from their natural source and solubilised, e.g. in an aqueous medium. Usually, such extraction/solubilisation involves at least disruption of the disulphide bonds (C), which breaks up the microfibrils (B) to provide the separate(d) keratin molecules (A). Depending upon the conditions used, the extraction/solubilisation may also involve hydrolysis/degradation of the keratin molecules (A) themselves, i.e. cleavage of the peptide bonds between the amino acids that form the keratin molecule(s) (A). Disruption of disulphide bonds has reportedly been achieved by oxidation of the disulphide bonds with organic peracids to form sulphonic acid groups; by sulphitolysis of the disulphide bonds to form S-sulphonate groups; or by reduction of the disulphide bonds with thiol compounds such as 2-mercaptoethanol, dithiothreitol (DTT) or dithioerytritol, or by treatment with alkali metal sulphides such as a sodium sulphide solution. Generally, the purpose of the prior art processes is to provide a keratin-derived product that is soluble in aqueous media, e.g. a solution of keratin(s) or keratin hydrolysate. For this purpose, the art describes modification of the disrupted disulphide bonds so as to avoid reformation of the bonds and/or the use of specific additives to keep the keratins in solution or to otherwise stabilise the keratin solutions, such as alkali metal hydroxides, urea, guanidine hydrochloride, 2-mercaptoethanol or thioglycolate.
E.g. U.S. Pat. No. 3,464,825 describes a process in which keratins are extracted from feathers using an alkali metal sulphide solution, such as a Na2S-solution. The keratin-solution thus obtained is then treated with an alkali metal sulphite such as a Na2SO3, after which the protein is acid-precipitated. The precipitated keratin is then solubilised in a aqueous alkali and subsequently oxidised using a water soluble oxidising agent, e.g. hydrogen peroxide, sodium periodate, sodium chlorite or an organic peracid such as peracetic acid or performic acid, which is believed to oxidise the cystine/cysteine residues to cysteic acid groups, However, the thus obtained keratins are essentially completely modified, i.e. all free cystine/cysteine residues are oxidised to cysteic acid groups. As a result the completely modified keratins are generally water-soluble and, therefore, less suited to provide water-insoluble films, e.g. for applications in coatings and for other applications mentioned hereinbelow. Also FR 2 522 657 describes essentially complete modification, e.g. of at least 70% of the free —SH groups of the keratins.
In contrast, U.S. Pat. No. 3,642,498, describes a process for the solubilisation of keratins using sodium sulphide whereby the cystine/cysteine groups remain essentially unmodified. The keratins are again extracted using an a metal sulphide solution, treated with an alkali metal sulphite solution, and then acid-precipitated. The resulting protein product is described as being dispersible in water-alcohol mixtures, and can be used for preparing films and coatings. U.S. Pat. No. 3,642,498 also describes alternative processes for extracting and solubilising feather keratins, including treatment with mercaptoethanol-alcohol-water mixtures; or treatment with alkaline mercaptoethanol-alcohol-water mixtures containing alkali metal hydroxides. However, according to U.S. Pat. No. 3,642,498, the cystine/cysteine groups remain essentially unmodified. A major disadvantage of solubilised keratins with essentially unmodified cystine/cysteine is that they do not allow to produce keratin-based products, in particular films and coatings, with the desired mechanical properties. In particular, such films suffer from brittleness.
Most of the art mentioned thus far dates back to the late 1960's and early 1970's. Nevertheless, in the 30 years since, these references have not led to any widespread use of the keratin-derived products disclosed therein. This is probably because the products and processes described are not economically viable—despite the fact that they employ waste feathers as a starting material—and/or because the keratin products obtained do not show the properties required for practical (e.g. commercial) use.
Some of these problems are addressed in the thesis of one of the inventors (P. Schrooyen; “Feather keratins: modification and film formation”, 1999, Thesis University of Twente, Enschede, The Netherlands) that describes keratin-derived products obtained by partial modification (i.e., alkylation) of intact feather keratins using monoiodoacetamide, monoiodoacetic acid or monobromosuccinic acid in concentrated aqueous urea solution and in the presence of 2-mercaptoethanol. The keratins were modified to degrees of modification varying between 25 and 87%, calculated on the basis of the amount of remaining free —SH groups. This partial modification provided stable dispersions of essentially intact (i.e. non-hydrolysed), partially modified keratins, which could be used to cast strong films with desired thermal and mechanical properties. However, extraction and solubilisation of essentially intact keratins for partial modification requires the use of high concentrations of chemicals such as urea and 2-mercaptoethanol. The use of these chemicals at experimental scale is acceptable. However, their use at large scale is not economically feasible because the use of these chemicals is expensive, also in view of the environmental and occupational hazards associated with the use of these chemicals, requiring expensive precautionary measures.
Thus, there is still a need for an economically viable method for the processing of keratin-containing (waste) materials such as feathers, which method can be used to provide a range of keratin-based products, in particular films and coatings, with properties acceptable for practical/commercial application(s).